Means for controlling the operation of hydraulically operated jacks

ABSTRACT

MEANS FOR CONTROLLING THE OPERATION OF A HYDRAULIC JACK HAVING LOAD SENSING MEANS IN WHICH A DUMPING VALVE AND A CHECK VALVE ARE ARRANGED IN THE HYDRAULIC CIRCUIT SO THAT, IN USE, WHEN THE LOAD SENSING MEANS REGISTER A PREDETERMINED VALUE, THE FLOW OF FLUID TO AND FROM THE RAM IS IMMEDIATELY STOPPED.

United States Patent Hugh Jeremy Willis Edwards Cobham, Surrey, EnglandJan. 21, 1969 June 28, 1971 Cable Covers Limited. Surrey. EnglandInventor Appl. No. Filed Patented Assignee MEANS FOR CONTROLLING THEOPERATION OF HYDRAULICALLY OPERATED JACKS 9 Claims, 2 Drawing Figs.

US. Cl. 91/1, 91/420, 91/448, 91/459 Int. Cl ..F0lb 25/26, F15b 15/04,E040 3/26 Field of Search 91/459,

[56] References Cited UNITED STATES PATENTS 1,898,228 2/1933 Thompson 9l/450X 3,074,384 1 1963 Pilch 91/420 3,469,399 9/1969 Krantz 9 l/459XPrimary Examiner-Martin P. Schwadron Attorney-Baldwin, Wight and BrownABSTRACT: Means for controlling the operation of a hydraulic jack havingload sensing means in which a dumping valve and a check valve arearranged in the hydraulic circuit so that,

in use, when the load sensing means register a predetermined value, theflow of fluid to and from the ram is immediately stopped.

MlEAWb W'Olii CONTIMJILMNG THE OPERATIION F iiiWlUtliilAUHCALlLYOPEWATEID JACMS This invention relates to means for controlling theoperation of hydraulically operated jacks particularly of the kind usedfor tensioning wires, rods or strands in prestrcssed concretestructures.

it has previously been proposed, for example, in our prior British Pat.Specification No. l,066,7 l 3 to provide prestressing jacks with loadcells or other load sensitive devices which, when coupled to a suitablemeter, are capable of measuring the stressing load applied by the ram towithin gor -1 percent which is the generally accepted degree of accuracyrequired when carrying out stressing operations. It is also ltnown fromthe aforementioned specification that switch means may be operativelyassociated with the load cell in order to close the main control valveof the hydraulic fluid system of the ram when the stressing load reachesa predetermined value.

ln order to achieve the generally accepted accuracy mentioned above, itis necessary to halt the operation of the jack immediately on attainingthe predetermined stressing load and this, in a prestressing jaclt underload, means that the supply of fluid to the jack must be accuratelycontrolled. It has been found that, by attempting to control the mainscontrol valve a time lag is encountered in changing the direction offlow of the fluid to cause it to return to the fluid reservoir with theresult that the desired degree of accuracy is not always attained.

it is among the objects of the present invention to provide means whichavoids or substantially reduces the above mentioned disadvantage.

According to the invention furthermore, means for controlling ahydraulically operated jaclr having, or having associated therewith, adirectional control valve through which fluid is passed from a reservoirvia a pump to either side of the ram piston, and a load cell or otherload sensitive device, which means comprise a dumping valve arranged inthe hydraulic circuit between the pump and the control valve, switchmeans adapted to operate the dumping valve to bypass the pumped fluidwhen the load cell registers a desired load, and a checlr valve which ispositioned in the hydraulic circuit to prevent return flow ofpressurized fluid from the jack.

The invention is illustrated, as applied to a prestressing jack for usein tensioning wires, rods or strands in prestressed concrete structures,in the accompanying drawings in which:

FlG. t is a longitudinal cross section partly in elevation through oneform of hydraulic jack to which the invention can be applied, andshowing schematically the hydraulic circuit for effecting controlthereof, and

FIG. 2 is an electric circuit diagram.

Referring to the drawings, there is shown a prestressing ram comprisinga stressing tube l at one end of which is provided a piston 2 havingseals 3. The other end of the tube 1 has connected thereto, by screwthreading a nose cone d within which is slidably received a barrelmember 5 having a tapered bore 6. The bore if: has arranged therein aplurality of radially disposed wedge elements 7 which are urged into anoperative position by means of a compression spring 8 supported by awedge cap 9. The nose cone 5 also has arranged therein a wedge releasetube M which is adapted to engage the smaller ends of the wedge elements7.

The nose cone 4 has attached thereto a lock-off cylinder 11 within whichis mounted a lock-off plunger 12. The free end of the cylinder it isadapted to receive a stressing nose l3.

Arranged within the stressing tube 1 is a center tube 14 which, at oneend, is attached by screw-threading to the barrel member 5 and is lockedrelative thereto by a locking key 15. The center tube M, which isaxially slidable within the tube 1, is provided adjacent said one endwith an annular seal 16 held in position by a retaining ring 17.

The center tube M passes through the piston 2 and the other end thereofhas attached thereto, by screw threading, an extension tube H8 providedwith a rear nut 19.

Suitably mounted on the stressing tube l is a main cylinder 20 one endof which is provided with a sealing ring 2B. The other end of the maincylinder 20 is provided with an end cap 22 and a sealing ring 23, theend cap 22 having flsed thereto, by bolts 24, a center tube retainingcap 25.

An annular load cell 26 is arranged between the cap 25 and the rear nutl9 so that stressing forces applied to the center tube is by movement ofthe main cylinder 20 are transmitted through the load cell.

The main cylinder 20 is provided with a stressing or extending inlet 27which communicates with an annular chamber 2b and receives hydraulicfluid under pressure via a line 29. The main cylinder is also providedwith a return or retracting inlet 30 to which is attached a fluid lineI. The inlet communicates with an annular chamber 32 provided betweenthe main cylinder 20 and the stressing tube 1 and, via a passageway 33,with an annular chamber 3d provided between the stressing tube 1 and thecenter tube M.

In use, the jack is positioned over the wire or the like to be stressedso that the wire extends through the plunger t2, the wedge release tube10, the wedge elements 7 and into the center tube M, and so that thestressing nose is in contact with the bearing plate of the concretestructure. Hydraulic fluid is then passed, via the line 29, into thechamber 28 whereby the main cylinder 20 will be moved outwardly relativeto the stressing tube 1. This movement will be transmitted, via the loadcell 26, to the center tube 14 so that the latter, together with thebarrel 5 containing the wedge elements, is also moved outwardly relativeto the stressing tube i.

Movement of the barrel 5 will cause the wedge elements 7 to move awayfrom the wedge release: tube 10 and allow the wedge elements 7, underthe action of the spring 8, to engage and grip the wire to be stressed,Further movement will tension the wire until a predetennined load, whichis governed by the load cell 26, is applied thereto at which stagemovement will cease. in this position, wedges associated with thebearing plate are forced home by means of the lock-off plunger 12 sothat the tensioned wire is held in position in the concrete structure.in order to release the wire, fluid under pressure from the line 31 ispassed into the inlet 30 to move the center tube M and thus also themain cylinder 20 inwardly. This movement causes the wedge elements 7 toengage the release tube 10 and thereby release their grip on the wire.

The operation described above is, in accordance with the invention,controlled as hereinafter described.

As can be seen from FIG. 1, the hydraulic control circuit comprises apump 35 which passes fluid under pressure from a reservoir 36 through adumping valve 37 which is operated by a solenoid 3d. The dumping valveis operable to bypass the fluid through an exhaust valve indicated at 39or to pass it, via line 40, to a directional control valve M.

The directional control valve ill is operable to pass the fluid throughline 29 to the stressing inlet 27 to extend the jack, or through line 3!to the return inlet 30 to retract the jack. The spool of the controlvalve 41 is moved between its operative positions by means of a solenoid42 which operates against the action of a return spring 43 which biasesthe spool into a position where the valve is operable to retract theram.

The lines 29 and 31 have positioned therein a pilot unloaded checltvalve assembly 44! consisting of a check valve d5 arranged in the line29 and pilot valve as in communication with the line 31, the plunger 417of the pilot valve as being operable, when required, to unseat the checkvalve 45.

A pressure relief valve 48 is arranged in communication with the line40.

As can be seen in FIG. 2, the electric control circuit comprises a lowvoltage, for example l2 volt, power supply 49 which receives power froma mains input line 50. Low voltage power is passed, via line 51, to oneside ofa selector switch 52 of the gang type which is adapted to bemoved between off, manual (MAN) and automatic (AUTO) positions asindicated on the drawing.

The other side of the gang switch 52 is coupled to lines 53 and 54 whichare returned to the transformer 49 to complete the circuit. Line 53includes a manual pushbutton switch 55 which is operable to causeextension of the ram and a relay RE which operates a switch SW1. Line 54includes a manual pushbutton switch 56 which is operable to causeretraction of the ram and a relay R2 which operates a switch SW2.

The switch SWl is connected via line 57 to the solenoid d2 of thehydraulic directional control valve M, and via line 58 to the solenoid38 of the hydraulic dumping valve 37. The switch SW2 is connected to themains supply 50.

The transformer 69 also supplies, via line 59, low voltage power to theload cell 26 and, via line 60, to an amplifier 6t connected to a meter62 which registers the load applied to the load cell 26. The readingregistered by the meter 62 may also be recorded on a recorder device ofany convenient type indicated at 63.

The system so far described is all that is necessary for manualoperation where the operator controls the operation by observing thereading registered by the meter 62.

In order to provide for automatic operation, the automatic contacts ofthe switch 52 are connected, via a line 64, to a trip device 65 having aswitch SW3 and a relay R3. The trip device is also provided with anadjustable trip level controller 66 and is connected to the meter 62 vialine 67 so that, when a load value determined by the position of thecontroller 66 is reached, the relay R3 operates to open the switch SW3to deenergize the circuit.

In use, for manual operation, the pump 35 is switched on and theselector switch 52 is set at manual. This energizes the relay R2 whichcloses switch SW2 thereby energizing, via line 58, the solenoid 38 ofthe dumping valve 37 so that fluid passes through the exhaust valve 39thereof and no movement of the ram will take place.

In order to extend the ram, the button 55, which is normally open, isdepressed so as additionally to energize relay R1 thereby causingchangeover of the switch SWil so that, via the line 57, the solenoid 42of the directional control valve 41 is energized. This automaticallydeenergizes the solenoid 32 of the dumping valve 37 and causes thepreviously bypassed fluid to be passed via line 40, to the control valveM. Energization of the solenoid 42 against the action of the spring 413causes the spool of the valve 45 to move into a position where the fluidpasses through the pilot unloaded check valve assembly 44 and via line29 to the chamber 28 on one side of the ram piston 2. As the ramextends, fluid on the opposite side of the ram piston 2, will return viathe line 31 and the valve assembly 44 to the control valve 411. Thisfluid is not under pressure and therefore no reaction occurs, and thefluid merely passes the exhaust through valve 68 of the directionalcontrol valve 4B.

The operator continues to depress the button 55 until the required loadis registered on the meter 62. On reaching the required load, the button55 is released thereby deenergizing the relay R1 and thus energizing thedumping valve solenoid $3 and deenergizing the control valve solenoid42. This causes the fluid to be bypassed through the exhaust valve 39and relieves the fluid pressure in the circuit. This lack of pressurecauses a drop in pressure in the line 29 and results in immediateseating of the check valve Q thus preventing fluid under pressure fromescaping from the chamber 28 of the ram. Thus, the applied load willremain static.

In order to retract the ram, the button 56, which is normally closed, isdepressed thereby deenergizing the relay R2 and causing changeover ofthe switch SW2. This cuts off the power to, and thus deenergizes, thesolenoids 38 and 42 with the result that the previously bypassed fluidis passed via line 40 to the directional control valve 431, and with theresult that the spool of the control valve 41 is moved to cause thefluid to flow through the line 31 to said opposite side of the rampiston 2.

In this position, fluid pressure will build up in order to retract theram, but initially, due to the fact that fluid is held on the extendside of the ram by the check valve 435, retraction will not take place.The pressure buildup continues until it reaches a predetermined valvewhich is sufficient to operate the pilot valve 46. At this point, theplunger 47 of the valve 46 is moved to unseat the check valve 45 wherebyfluid flows through the line 29 to the control valve M from which itpasses to exhaust through valve 68.

The button 56 is held in its depressed position until the jack isvisibly at the end of its stroke at which stage it is released. Releaseof the button 56 causes the relay R2 to be energized thus energizing thesolenoid 33 which results in the fluid again being bypassed through theexhaust valve 39 of the dumping valve 37. The operation can then berepeated.

in use, for automatic operation, the selector switch 52 is set atautomatic. Thereafter the operation is basically the same as thatdescribed with reference to manual operation.

In this case however, power for relay R1 is routed through line 64 tothe relay R3 of the trip device 65 which relay is deenergized at apreset trip point determined by the setting of the trip level controller66. Thus, as hereinbefore described, depression of the button 55 toextend the ram causes the dumping valve solenoid 38 to be deenergizedand the control valve solenoid to be energized. Extension of the ramcontinues until a given load value determined by the controller 66 isreached. At this point, the relay R3 is energized thereby opening theswitch SW3 which cuts off the power to, and deenergizes, relay R1. Thiscauses the control valve solenoid 42 to be deenergized and the dumpingvalve solenoid 38 to be energized.

The latter operation overrides the button 55 which is still in thedepressed position. On releasing the button 55, the dumping valvesolenoid 38 would normally be energized but as this has already beeneffected by the trip controller, there is no further change.

In order to retract the ram, the button 56 is depressed and furtheroperation is as hereinbefore described. The relay R3 is deenergized toclose switch SW3 when the applied load decreases below the value set bythe controller 66.

Thus, it will be appreciated that, by including the dumping valve 37 inthe hydraulic circuit, an immediate response to the signal passed out bythe load meter 62 is achieved and the time lag resulting from changingover the main control valve 41 can be ignored from the time factor pointof view. Furthermore, the inclusion of the check valve 45 ensures that,as soon as fluid pressure is released by the dumping valve 37, thepressure in the ram remains constant so that the desired predeterminedstressing load applied to the wire is maintained and held.

The inclusion of the recording device 63, which may be of thepen-recorder-type makes it possible permanently to record the stressingload attained, it being understood that records ofthis kind would beparticularly useful when carrying out large operations such as arerequired for example in nuclear power stations, and in providing qualitycontrol which is necessary when carrying out pretensioning work.

Although the invention has been described with reference to a jack forprestressing the wires or the like in a concrete structure, it will beunderstood that it is not limited in this respect. Thus, the inventioncan, without modification, be applied to any hydraulically operated ramhaving a load cell or other load sensitive device and control means fordetermining the direction of operation of the ram.

lclaim:

1. Improved control means for controlling the operation of ahydraulically operated jack including a ram comprising a piston, andhaving associated therewith a directional control valve, a pump forpassing hydraulic fluid from a reservoir to either side of the piston ofsaid ram via said directional control valve, and a load cell for sensingthe load applied by said ram, the improvement comprising a dumping valvearranged in a hydraulic circuit between said pump and said directionalcontrol valve, switch means for operating said dumping valveindependently of said directional control valve to bypass fluid beingpumped toward said ram, said switch means being controllably coupled tosaid load cell for actuating said dumping valve in response to said loadcell sensing a given desired load, and a check valve positioned in saidhydraulic circuit to prevent return flow of pressurized fluid from saidjack upon actuation of said dumping valve.

2. Improved control means for controlling the operation of ahydraulically operated jack including a ram comprising a piston, andhaving associated therewith a directional control valve, a pump forpassing hydraulic fluid from a reservoir to either side of the piston ofsaid ram via said directional control valve, and a load cell for sensingthe load applied by said ram, the improvement comprising a dumping valvearranged in a hydraulic circuit between said pump and said directionalcontrol valve, switch means including a manually operated relayactuatedchangeover switch for operating said dumping valve independently of saiddirectional control valve to bypass fluid being pumped toward said ram,load indicating means for providing an indication of said load cellsensing a desired load, and a check valve positioned in said hydrauliccircuit to prevent return flow of pressurized fluid from said jack uponactuation of said dumping valve.

3. improved control means as claimed in claim 2, wherein said indicatingmeans comprises a meter coupled to said load cell for providing a visualindication of the load sensed by said load cell.

4. Improved control means as claimed in claim 3, including a recordingdevice for providing a record of loads sensed by said load cell duringthe course of a plurality of operations.

5. improved control means as claimed in claim 2, including a pilot valvefor unseating said check valve to permit return flow of pressurizedfluid from said jack.

6. Improved control means for controlling the operation of ahydraulically operated jack including a ram comprising a piston, andhaving associated therewith a directional control valve, a pump forpassing hydraulic fluid from a reservoir to either side of the piston ofsaid ram via said directional control valve, and a load cell for sensingthe load applied by said ram, the improvement comprising a dumping valvearranged in a hydraulic circuit between said pump and said directionalcontrol valve, a manually operated relay-actuated changeover switch foroperating said dumping valve independently of said directional controlvalve, a trip device controllably coupled to said load cell and arrangedin circuit with said changeover switch for automatically actuating saidchangeover switch in response to said load cell sensing a predeterminedload, and a check valve positioned in said hydraulic circuit to preventreturn flow of pressurized fluid from said jack upon actuation of saiddumping valve.

7. Improved control means as claimed in claim 6, in which said tripdevice comprises a relay-operated cutout switch and an adjustable triplevel controller which determines the load value at which said cutoutswitch becomes operative.

8. Improved control means as claimed in claim 6, including a meter forproviding a visual indication of the load sensed by said load cell.

9. Improved control means as claimed in claim 6, including a pilot valuefor unseating said check valve to permit return flow of pressurizedfluid from said jack.

